In an elevator system, the movement and the position of an elevator car are registered by means of sensor devices. Typically foreseen in such cases is that also a possible faulty behavior of the elevator system, for example the occurrence of overspeed of the elevator car, is detected, so that the required safeguarding measures can be initiated.
A method and an apparatus for measuring the speed, and for detecting overspeed, in an elevator system are described in EP 0 712 804 A1. By means of this known apparatus, the travel speed of an elevator car that is guided in an elevator hoistway, and driven by a drive unit, is monitored, so as to bring it to a standstill should overspeed occur.
To this end, fastened to a wall of the elevator hoistway is a measuring strip, which is scanned by a fork-light-barrier that is connected to the elevator car. The measuring strip has a measuring track with vanes, with the aid of which the speed of the elevator car is measured. Consequently, by comparing the measured speed with the specified maximum speed, the possible occurrence of overspeed can be detected and signaled. The respective length of the vanes is adapted to the maximum speed of the elevator car in the corresponding area of the hoistway, i.e. towards the upper and lower ends of the hoistway, the vane segments become increasingly shorter. The scanning duration of the individual vanes therefore remains at an at least approximately constant limit value, provided that the entire hoistway area is traveled through with the foreseen maximum speed. Should the duration of the scanning of an individual vane be shorter than this limit value, an impermissible exceeding of the maximum speed has occurred.
The measuring strip further has a control track with window openings, each of which is assigned to, and arranged at the same height as, a vane. Provided that the measuring strip and the fork-light-barrier are correctly installed, the markings of the measuring track, and of the control track, will be correctly scanned. Hence, by scanning the window openings of the control track, it is checked whether the fork-light-barrier engages sufficiently deeply in the measuring strip, and whether the sequential interruption of the light-barrier, or usually a plurality of light-barriers, by the vanes during travel of the elevator car is assured. Through scanning of the control track, it can further be determined whether individual vanes on the measuring strip are missing, as a result of which the speed measurement would be falsified. The vanes of the measuring track, and the window openings of the control track, are dimensioned and arranged in such manner that always at least one light-barrier is interrupted. Hence, should the light-barriers that are assigned to the measuring track and the control track be simultaneously uninterrupted, a fault is present, such as occurs, for example, if the fork-light-barrier has separated from the measuring strip.
In a preferred embodiment of this known apparatus, the measuring strip has, in addition to the measuring track and the control track, a safety track, which serves to additionally monitor the elevator car in the upper and lower end-areas of the elevator hoistway.
The fork-light-barrier has further a first and a second optical channel with mutually independent light-barriers, whose signals are input to a first and a second measurement channel. Should the measurement results of these two measurement channels differ from each other, a fault is detected, which is attributable, for example, to failure of an individual optical component.
Despite these many and diverse safeguarding measures, under certain circumstances also in this apparatus, faults can occur which endanger the safe operation of the elevator system. For example, identical faults can occur in both channels of the fork-light-barrier. Further, damage to the measuring strips, or permanent effects of extraneous matter, can occur. Should the aforementioned impairments in the fork-light-barrier of the measuring strip occur, the markings of the measuring strip are no longer correctly scanned, as a result of which, correct measurement of the speed, and hence also detection of an overspeed, are no longer possible.
Also, under certain circumstances, the indicated states do not contain any direct, unequivocal information as to the true state of the elevator system. For example, a state can occur in which all of the light-barriers are interrupted by the measuring strip. This state can continue for a relatively long period of time, if the elevator car is brought to a standstill at a corresponding position inside the elevator hoistway. The same state can, however, also occur if the elevator car is traveling and one of the aforementioned faults occurs. Based on the available information, it is therefore not possible to determine unequivocally whether the elevator car is at a standstill at a certain position, or whether it is moving along the elevator hoistway.